Process and apparatus for the separation of air by cryogenic distillation

ABSTRACT

In a process for the separation of air by cryogenic distillation using a cryogenic distillation unit comprising at least a double column ( 1 ) including a high pressure column ( 2 ) and a low pressure column ( 3 ), in which air is compressed in a compressor (C) to a first pressure, cooled and purified air at the first pressure is compressed in first and second booster compressors ( 8, 11 ) to a second pressure and then cooled in a heat exchanger ( 5 ), at least part of the air at the second pressure is cooled and expanded in a first turbine ( 12 ) having a first inlet temperature, a first part of the air expanded in the first turbine is sent to the high pressure column and a second part of the air expanded in the first turbine is sent to the heat exchanger to be warmed, the warmed second part of the air is expanded in a second turbine ( 9 ), returned to the heat exchanger and further warmed and a portion of the air is compressed in the first booster compressor ( 8 ) to a pressure intermediate to the first and second pressures, sent to the heat exchanger, cooled, liquefied, and sent to at least one column of the double column.

This application claims the benefit of priority under 35 U.S.C. §119 (a) and (b) to Indian Application No. 606/CHE/2005, filed May 20, 2005, the entire contents of which are incorporated herein by reference.

BACKGROUND

This invention relates to a process and apparatus for the separation of air by cryogenic distillation. It applies to the separation and liquefaction of the gases found in the air and to the apparatus for this distillation of air. It is concerned with processes using refrigeration production by expansion of air in two turbines. Part of air is expanded in a first turbine called cold turbine followed by expansion of a portion of the fluid originating from this first turbine in a second turbine called warm turbine.

In U.S. Pat. No. 5,157,926, all the compressed air is boosted up in two boosters connected in series. It is then cooled and expanded in high pressure turbine, after which a portion of this air is expanded again in a low pressure turbine.

This arrangement limits the liquid to gas ratio in large air separation plants, because of machine unavailability. There is a large disparity in flows through warm booster and warm expander.

The invention aims at providing a process enabling to improve refrigeration production.

According to the invention, there is provided a process for the separation of air by cryogenic distillation using a cryogenic distillation unit comprising at least a double column including a high pressure column and a low pressure column, in which air is compressed in a compressor to a first pressure, cooled and purified air at the first pressure is compressed in first and second booster compressors to a second pressure and then cooled in a heat exchanger, at least part of the air at the second pressure sent to the heat exchanger, cooled, liquefied and sent to at least one column of the double column and at least a portion of the air is compressed in the first booster compressor to a pressure intermediate the first and second pressures, is cooled and expanded in a first turbine having a first inlet temperature, a first part of the air expanded in the first turbine is sent to the high pressure column and a second part of the air expanded in the first turbine is sent to the heat exchanger to be warmed, the warmed second part of the air is expanded in a second turbine, returned to the heat exchanger and further warmed.

According to further optional aspects of the invention:

-   -   all the air compressed to the intermediate pressure and then         cooled is sent to the first turbine;     -   at least one liquid product is produced;     -   the first booster compressor is coupled to the first turbine and         the second booster compressor is coupled to the second turbine.

According to a further aspect of the invention, there is provided an apparatus for the separation of air by cryogenic distillation, the apparatus comprising a double column comprising a high pressure column and a low pressure column, a compressor, first and second turbines, first and second booster compressors and a heat exchanger, a conduit for sending air at a first pressure from the compressor to the first and second booster compressors connected in series, a conduit for sending air at a second pressure from the outlet of the second booster to the heat exchanger and thence to at least one column of the double column. A conduit for removing air from the first booster compressor and a conduit for sending the air removed from the first booster to the heat exchanger and thence to the first turbine, a conduit for sending air from the first turbine to the high pressure column, a conduit for sending air from the first turbine to a cold end of the heat exchanger, a conduit for removing the air sent from the first turbine to the heat exchanger from an intermediate point of the heat exchanger, a conduit for sending the removed air to a second turbine and a conduit for returning air from the second turbine to the heat exchanger.

There may be means for sending all the air compressed in the first booster compressor to the first turbine.

The first booster compressor may be coupled to the first turbine and the second booster compressor is coupled to the second turbine.

The apparatus may comprise an argon column and means for feeding an argon enriched stream from the low pressure column to the argon column.

Examples of operating the invention will now be described with reference to the annexed drawings on which:

FIG. 1 is a schematic view of an apparatus for distillation of air according to the invention.

FIG. 2 is a heat exchange diagram for a cycle of liquefaction according to the invention.

The apparatus for the distillation of air represented in FIG. 1 is intended to produce oxygen, nitrogen and argon in gaseous and liquid form. It comprises a double distillation column 1, the latter comprising a high pressure column 2 operating at about six bars absolute, which is surmounted by a low pressure column 3, operating slightly above atmospheric pressure. The gas in the head portion (nitrogen) of column 2 is in indirect heat exchange relationship with the liquid in the vat portion (oxygen) of the column 3 by means of a vaporizer-condenser 4.

The apparatus also comprises a heat exchange line 5 with counter-current circulation of the fluids in heat exchange relationship, and two turbine-booster units 6 and 7. Unit 6 comprises a booster 8 and a warm low pressure turbine 9 mounted on the same shaft 10, and unit 7 comprises a booster 11 and a cold high pressure turbine 12 mounted on the same shaft 13. The two boosters 8 and 11 are mounted in series.

The air to be separated, compressed to about 20 bars abs. in compressor C and free from water and CO₂ following purification in purification unit A is boosted at about 32 bars by the first booster 11. The stream is then split in two. A first portion P1 of the air is sent to the heat exchange line and cooled down to a temperature T1, for example of the order of −125° C.; in ducts of the exchange line 5. It is then taken out of the exchange line 5 through duct 17 and is expanded down to 6 bars abs in turbine 12 from which it exits at about its dew point. A portion, for example about one quarter, of this air may continue to be cooled until reaching the cold end of the heat exchange line 14.

A second portion P2 of the air is further compressed in the second booster 8 to a pressure of 38 bars and then cooled by passing from the warm end to the cold end thereof in ducts 14 from which it exits in liquid state, after which, via duct 15, it is expanded down to six bars in an expansion valve 16 and is injected at the bottom of the high pressure column 2. This liquid air can be expanded down to the low pressure and injected into the column 3.

All the air (or the remaining air) at a pressure of 38 bars.

A portion of air which originates from the turbine 12, corresponding for example to about 1/10^(th) of the initial air stream compressed to 20 bars abs., is sent to the vat portion of column 2 through conduit 18 and the remaining portion is warmed up in ducts 19 of the exchange line, from the cold end of the latter to a temperature T2 which is much higher than T1. This temperature T2 may for example be between room temperature and about −20° C.

The air thus warmed up is taken out of the exchange line via duct 20 and is expanded up to about atmospheric pressure in turbine 9, from which it exits at a temperature in the vicinity of T1. It is thereafter reintroduced into the exchange via duct 21, warmed up to room temperature in ducts 22 and is evacuated from the apparatus, after having eventually been used to refrigerate an adsorbent used for purifying incoming air and/or to cool outgoing air from the main compressor of the apparatus.

As a variant, as represented in FIG. 1 all of the air which originates from turbine 9 can be warmed until reaching the warm end of the exchange line and then is sent to the atmosphere.

The remaining portion of the apparatus is well known: the rich liquid LR (oxygen enriched air) collected in the vat portion of the column 2 is sent into column 3 after sub-cooling in a sub cooler 25 by heating the residual nitrogen coming from the top of the low pressure column 3, after which it is expanded in an expansion valve 26, and poor liquid LP essentially consisting of nitrogen, withdrawn in the upper portion of column 2, is also sent into column 3 after sub-cooling in a sub-cooler 25 after which it is expanded in an expansion valve 28. The apparatus produces liquid nitrogen, taken up in the head portion of the column 2 via duct 29, which is sub-cooled in sub-cooler 25, expanded down to about atmospheric pressure in an expansion valve 30 and stored in a container 31. Liquid oxygen is taken up in the vat portion of column 3 via a duct 32 and sub-cooled in a sub-cooler 25. The latter is cooled by means of residual nitrogen withdrawn in the head portion of column 3 via a duct 34. Oxygen in the form of vapour 35 withdrawn from the bottom of column 3 is warmed up in the main exchanger to cool the incoming air. Another product, low pressure nitrogen 37 taken from the top of column 3, is also passed through sub-cooler and main exchanger to cool the other incoming gases and liquids.

The process also includes a conduit 38 for sending an argon-enriched feed stream from the low pressure column 3 to an argon column 39. The argon is further purified in a nitrogen removal column 41. This part of the process is entirely standard and is not described in detail.

With reference to FIG. 2 where the temperature in ° C. has been shown on the x axis and the heat flow, is given on the y axis, the lower curve C1 represents the variation of heat flow of the air being cooled and liquefied, and the upper curve C2 represents the variation of heat flow of the gas being warmed up. It will be seen that:

The cold turbine 12 treats a high flow of air with inlet and outlet temperatures which border the liquefaction zone of the air, i.e. it produces much more cold in spite of its operation at low temperature, moreover it produces this cold in a temperature zone where precisely a lot of refrigeration is required to liquefy the air and where, on the other hand, heat losses are maximum, and the hot turbine treats a small flow of air and may recover, by ensuring an expansion from 6 bars to 1 bar, the essential of the temperature zone located above the previous one and in which the cooling is ensured by the turbines. So the turbine 9 produces a small amount of refrigeration over a wide zone of temperature.

It results from the above considerations that the apparatus of FIG. 1 leads to a reduced specific energy of liquefaction. It will also be noted that the air at high pressure which circulates in duct 18 may without inconvenience be in the vicinity of its dew point which is of interest for distillation in the double column.

It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above. 

1. A process for the separation of air by cryogenic distillation using a cryogenic distillation unit comprising at least a double column including a high pressure column and a low pressure column, in which air is compressed in a compressor to a first pressure, cooled and purified, air at the first pressure is compressed in first and second booster compressors to a second pressure and then cooled in a heat exchanger, at least part of the air at the second pressure sent to the heat exchanger is cooled, liquefied and sent to at least one column of the double column and at least a portion of the air is compressed in the first booster compressor to a pressure intermediate the first and second pressures, is cooled and expanded in a first turbine having a first inlet temperature, a first part of the air expanded in the first turbine is sent to the high pressure column and a second part of the air expanded in the first turbine is sent to the heat exchanger to be warmed, the warmed second part of the air is expanded in a second turbine, returned to the heat exchanger and further warmed.
 2. The process of claim 1, in which all the air compressed to the intermediate pressure and then cooled without further compression is sent to the first turbine.
 3. The process of claim 1, in which at least one liquid product is produced.
 4. The process of claim 1, in which the first booster compressor is coupled to the first turbine and the second booster compressor is coupled to the second turbine.
 5. An apparatus for the separation of air by cryogenic distillation, the apparatus comprising a double column comprising a high pressure column and a low pressure column, a compressor, first and second turbines, first and second booster compressors and a heat exchanger, a conduit for sending air at a first pressure from the compressor to the first and second booster compressors connected in series, conduits for sending air at a second pressure from the outlet of the second booster to the heat exchanger and thence to at least one column of the double column, a conduit for removing air from the first booster compressor and a conduit for sending the air removed from the first booster to the heat exchanger and thence to the first turbine, a conduit for sending air from the first turbine to the high pressure column, a conduit for sending air from the first turbine to a cold end of the heat exchanger, a conduit for removing the air sent from the first turbine to the heat exchanger from an intermediate point of the heat exchanger, a conduit for sending the removed air to a second turbine and a conduit for returning air from the second turbine to the heat exchanger.
 6. The apparatus of claim 5, in which the outlet of the first booster compressor is connected only to inlet of the first turbine.
 7. The apparatus of claim 5, in which the first booster compressor is coupled to the first turbine and the second booster compressor is coupled to the second turbine. 